The present invention is directed to a method and apparatus for detecting hazardous thunderstorm outflows such as microbursts, using airport surveillance radars.
Low level wind shear resulting from microbursts, wind gust fronts, tornado vortices, mesocyclones or other thunderstorm phenomena, are particularly hazardous to aircraft during takeoff and landing. These hazardous weather disturbances may be detected by specially dedicated narrow beam Doppler weather radars. However, these specialized radar systems are extremely expensive and are not available at most airports. Therefore, it is an object of the present invention to use airport surveillance radar which is readily available at most airports in the detection of low level wind shear.
U.S. Pat. No. 4,649,388 (Atlas) discloses a method for using airport surveillance radar to detect hazardous weather disturbances. This method uses the high and low beam antenna receivers on the airport surveillance radars. Use of this dual-beam signal processing approach, however, raises several implementation issues. ASRs normally transmit circularly polarized (CP) signals during heavy rain in order to reduce precipitation clutter in the aircraft detection processor. In order to avoid a resulting 15 to 20 dB loss in received power from weather echos, any weather processor on an airport surveillance radar should receive its input from the opposite-sense polarized antenna port. ASR-8's and ASR-9's have only one path through the rotary joint of the radar for opposite-sense polarized signals; thus during operation with circular polarization, weather data from both high and low beams could not be accessed simultaneously. Power spectra for the two beams would have to be calculated on alternate antenna scans, requiring memory storage for all data from one of the scans. Assuming range coverage to 20 km and two byte integer representation of the in-phase and quadrature signals, this would require 3 Mbytes of dedicated physical memory.
Another issue is the computational requirement of estimating power spectra for the two beams in each resolution cell and calculating a velocity based on the difference of the two spectra. Assuming that the spectra were estimated using Fast Fourier Transforms (FFT), about 1000 floating point operations per resolution cell would be required. For the same 20 km range coverage considered previously, this translates to 17 million floating point operations per second. While both the memory storage and processing speed requirements could be met, they would certainly drive the cost of the signal processing computer higher.
It is an object of the present invention to provide a means for detecting hazardous weather disturbances with reduced computational and storage requirements while maintaining the high-confidence created from dual-beam signal processing of ASR information.